gh-41094: Add some missing docstrings to `multi_polynomial_ring_base.pyx`
This PR adds a few missing docstrings and doctests to
`multi_polynomial_ring_base.pyx`.
While I am at it, I also fixed a typo in `scheme.py`.
- [X] The title is concise and informative.
- [X] The description explains in detail what this PR is about.
- [ ] I have linked a relevant issue or discussion.
- [X] I have created tests covering the changes.
- [X] I have updated the documentation and checked the documentation
preview.
URL: https://github.com/sagemath/sage/pull/41094
Reported by: Rubén Muñoz--Bertrand
Reviewer(s): David Coudert, Rubén Muñoz--Bertrand
